Multiple cubes as just one object?
What you're trying to accomplish is achieved in Edit Mode. You can enter the dropdown menu in the menus section of your header, or press Tab. Enter Edit Mode, select all the vertices of the cube, then use Shift-D to duplicate.

Ian Hubert's tutorials are for intermediate users. He operates under the assumption that you are already familiar with the basics of Blender. I recommend following a beginner-friendly tutorial series, then continuing where you left off.